Mississippi Valley-type mineralization from two major host formations in the central Appalachians was studied in order to gain a better understanding of depositional conditions, sources of ore constituents, and the distribution of mineralizing fluids in the region. Sulfur isotope data obtained in this study and from the literature (Ault and Kulp, 1960; Howe, 1981; Crawford and Beales, 1983; LeHuray, 1984; Wilbur et al., 1990) show that mineralization in each formation acquired sulfur from sources of similar isotopic composition. Barite deposits in the Beckmantown Group have (delta 34 S values ranging from 25 to 36 per mil; the values probably closely reflect the isotopic composition of the sulfur-bearing fluid involved in the barite formation. Sulfide deposits in this formation have more variable compositions and more complex isotopic systematics. In both the Timberville district and Nittany Arch area, main-stage sulfide minerals have delta 34 S values ranging from about 15 to 30 per mil. Each district also has a smaller, isotopically lighter substage of sphalerite mineralization that has (delta 34 S values between 1 and 19 per mil. Sulfate minerals in the Nittany Arch area are isotopically heavier than the sulfides, with delta 34 S values between 30 and 40 per mil. The lightest sulfur in the central Appalachian Mississippi Valley-type districts is found in the Friedensville district, where delta 34 S values of sulfide minerals range between -9 and +1 per mil. Sulfide minerals from the two Silurian Tuscarora-Shawangunk-hosted deposits have similar (delta 34 S values of 21 to 35 per mil. Minor amounts of sulfate minerals in this formation are isotopically heavy with delta 34 S values between 34 and 43 per mil.New fluid inclusion data were also obtained from the Timberville district and reveal the presence of at least two distinct fluids involved in mineralization-a more saline or calcium-rich fluid with final melting temperatures between -42 degrees and -29 degrees C, and a less saline or more calcium-poor fluid with final melting temperatures of -24 degrees to -12 degrees C. Homogenization temperatures of both inclusion types ranged for the most part between 100 degrees and 170 degrees C.Geologic evidence suggests that the Beckmantown Group and Tuscarora-Shawangunk Formations were part of distinct palcoaquifers, and together with the sulfur isotope data, also suggests that the source of sulfur for Mississippi Valley-type deposits in each palcoaquifer was cormate (formational) fluids derived from seawater, or alternatively in the case of the Beckmantown Group, intraformational evaporites. Furthermore, evidence of mixing and a temporal increase in the oxidation state of mineralizing conditions in at least two of the districts suggests that much of the Mississippi Valley-type mineralization in the central Appalachian region may be the result of mixing of oxidizing tectonically driven Zn-Pb or Ba-rich fluids with sulfur-rich fluids of varying oxidation state present in the Beckmantown Group and Tuscarora-Shawangunk Formations.